[PATCH v2 14/14] mm/shmem: fix truncation of files containing only swapin-error markers

["David Hildenbrand (Arm)" <david@kernel.org>]

It might not be very common, but we can end up with shmem files that are
composed only of swapin-error/poison markers.

Such markers are not counted in info->alloced. Consequently,
shmem_setattr() can skip shmem_truncate_range(), leaving the markers
behind.

Truncating the file to zero and growing it again will result in reads
failing with -EIO instead of returning zeroes.

Check whether the mapping contains entries, in addition to
info->alloced, before skipping the truncation. Keep the info->alloced
check so that shmem_recalc_inode(), called at the end of
shmem_undo_range(), can fix up accounting after reclaim has emptied the
mapping.

Fixes: 6cec2b95dadf ("mm/shmem: fix infinite loop when swap in shmem error at swapoff time")
Cc: stable@vger.kernel.org
Tested-by: Gregory Price (Meta) <gourry@gourry.net>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/shmem.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 2800dbdf031c1..3c42b4f253b5e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1591,7 +1591,8 @@ static int shmem_setattr(struct mnt_idmap *idmap,
 			if (oldsize > holebegin)
 				unmap_mapping_range(inode->i_mapping,
 							holebegin, 0, 1);
-			if (info->alloced)
+			/* Swapin-error markers are not counted in alloced. */
+			if (info->alloced || !mapping_empty(inode->i_mapping))
 				shmem_truncate_range(inode,
 							newsize, (loff_t)-1);
 			/* unmap again to remove racily COWed private pages */

-- 
2.43.0